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Repurposed Switch-Mode Power Supply 



i 



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Repurposed Switch-Mode Power 



Supply 

Written By: Ken Rich 



SUMMARY 

Building your own switch-mode power supply (SMPS) can be fraught with disappointment 
and frustration. Even with modern ICs designed to make SMPS design easier, you can still 
run into issues with PCB layout and circuit efficiency. This project demonstrates how you 
can build your own SMPS by using a charger you may have stuffed in a drawer somewhere. 

Details on this project and more: 

Building Blocks: Switch Mode Power Supply 

Ramblings of an E-Geek 



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Repurposed Switch-Mode Power Supply 





• First, you must find a car charger 
that you don't really want anymore. 
This is mainly because you will be 
tearing it apart and it will be 
useless for charging. 

• This is a picture of the type of 
charger that I had in the bottom of 
my junk drawer. The nice thing 
about these chargers is that the 
input voltage is 12V because it's 
standard car voltage, and the 
output voltage is 5V because it's 
standard USB voltage. Pretty 
sweet. 

• Warning... Some chargers simply 
use a 7805 or equivalent circuit. 
These are LDO regulators and just 
burn up the extra voltage as heat. 
That kind of defeats the purpose, 
no? 



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Repurposed Switch-Mode Power Supply 





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• Now, take the charger apart and you will find a couple of things that will positively identify 
the circuit as being an SMPS. 

• The first giveaway is the presence of an inductor. Most simple SMPS circuits make use of 
an inductor. 

• The next giveaway is the presence of a potential/voltage divider in addition to the inductor. 
The voltage divider is used to set the output voltage. 

• You can expect that this charger will use a very simple circuit, similar to the picture 
shown. 

• The LM2575 simple switcher by National Semi is a good example of what you might find 
inside your charger. In my case, the chip was unmarked, but when I traced the PCB, it 
matched the example application schematic for the LM2575 almost exactly. 



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Repurposed Switch-Mode Power Supply 




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Repurposed Switch-Mode Power Supply 



Adjustable Output Voltage Version 

FEEDBACK 




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R2 



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• Now, you might be done at this 
point if you want a 12V to 5V 
SMPS. All you need to do is solder 
on your own connectors to the 
input/output points and go for it. 

• If you need something other than 
5V output (like me), then you need 
to take one more step. 

• Notice the voltage divider on the 
output? You will need to replace 
one of those resistors based on 
what V out value you want. 

• The easiest way is to identify the 
value of R1 , either by color code or 
by multimeter, and then use the 
second equation in the picture to 
calculate R2. 

• For example: If I want to make this 
a 12V to 3.3V SMPS, my R1 value 
is 1k ohms, and my V re f is 1.23V, 

then my formula looks like this... 
R2=1000((3.3/1.23)-1) 

• Through the magic of the Internet, I 
have already calculated R2 for you 
and that is: 1682 ohms. 

• I confess that I don't remember 
what the actual value of my R1 
resistor was, but I do remember 
that my R2 resistor was 1000 
ohms. Working backwards, that 
must mean my R1 was about 600 
ohms. 



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Repurposed Switch-Mode Power Supply 




• I hope you found this project useful. I do have a couple of caveats to throw out there, 
though. 

• Remember, this circuit was engineered to supply 5V to charge a cell phone. Most cell 
phones can charge at a low rate and at a high rate due to the fact that they are charged by 
USB. 

• Basic USB 2.0 standards say that a device will be current-limited to 100mA unless a 
higher current is negotiated, in which case a maximum current of 500mA can be offered. 

• Why is this important? Well, I guess there's really no way to tell if this circuit can handle 
100mA, 500mA, or more. In my case, the markings on the chip were removed, so I was 
unable to look up any concrete information on it. 

• I guess what I'm saying is to be careful with it. It'll likely power your AVRs, PICs, 
Arduinos, etc., but I wouldn't try to run motors off it, or large LED arrays, etc. 

• For more information on this, visit my blog at: Ramblings of an E-Geek 

This document was last generated on 201 2-11 -01 1 0:01 :23 PM. 



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